A number of different color spaces have been used to describe the human visual system. In one attempt to define a workable color space, Commission Internationale de l'Eclairage (International Commission on Illumination) developed the CIE Chromaticity Diagram, first published in 1931. The CIE color model employed the tristimulus values X,Y,Z based on a standard human observer. In later work, the CIE Chromaticity Diagram in X, Y, and Z was modified to a u′ and v′ diagram in which equal distances on the diagram represent equal perceived color shifts. Useful background discussion of the CIE Chromaticity Diagram and of color perception and color models in general can be found in Billmeyer and Saltzmann's Principles of Color Technology, Third Edition, Wiley and Sons, and in chapter 7 of Dr. R. W. G. Hunt's The Reproduction of Color, Fifth Edition, Fountain Press, England.
FIG. 1 shows a familiar color gamut representation using CIE 1976 L*u*v* conventions, with the perceived eye-brain color gamut in u′-v′ coordinate space represented as a visible gamut 100. Pure, saturated spectral colors are mapped to the “horseshoe” shaped periphery of the visible gamut 100 curve. The interior of the “horseshoe” contains all mappings of mixtures of colors, such as spectral red with added blue, which becomes magenta, for example. The interior of the horseshoe can also contain mixtures of pure colors with white, such as spectral red with added white, which becomes pink, for example. The overall color area defined by the “horseshoe” curve of visible gamut 100 is the full range of color that the human visual system can perceive. It is desirable to represent as much as possible of this area in a color display to come as close as possible to representing the original scene as it would be perceived by a human observer.
Conventional motion picture display, whether for large-scale commercial color projection from film or for color television CRTs, operates within a fairly well-established color gamut. Referring again to the mapping of FIG. 1, observe that visible gamut 100 shows the full extent of human-perceivable color that, in theory, could be represented for motion picture display. A more restricted motion picture film gamut 102 is mapped out within visible gamut 100, showing the reduced extent of color representation achievable with conventional film media. A further restricted NTSC TV gamut 104 shows the limitations placed on achievable colors using conventional color CRT phosphors. It is instructive to note that, because the colors of the CRT phosphors for NTSC TV gamut 104 are not typically saturated, the points defining the color of each phosphor do not lie on the periphery of visible gamut 100. Hence, for example, colors such as turquoise and neon orange can be perceived by the eye in the actual scene but are beyond the color capability of a CRT phosphor system. As is clear from FIG. 1, the range of colors that can be represented using conventional film or TV media falls far short of the full perceivable range of visible gamut 100.
Conventionally, the component colors used for motion picture film employ red, green and blue dyes (or their complementary counterparts cyan, magenta and yellow) as primary colors. Component colors for color television CRTs employ red, green, and blue phosphors. These dyes and phosphors, initially limited in the colors that they could represent, have been steadily improved. However, as is clear from the gamut mapping represented in FIG. 1, there is still considerable room for improvement in approximating visible gamut 100 in both motion picture and TV environments.
With the advent of digital technology and the demonstration of fully digital projection systems, there is renewed interest in increasing the range or gamut of colors that can be displayed in order to provide a more realistic, more vivid image than is possible with the gamut limitations of film dyes or phosphors. The most promising solutions for digital cinema projection employ, as image forming devices, one of two types of spatial light modulators (SLMs). A spatial light modulator can be considered essentially as a two-dimensional array of light-valve elements, each element corresponding to an image pixel. Each array element is separately addressable and digitally controlled to modulate transmitted or reflected light from a light source. There are two salient types of spatial light modulators that are being employed for forming images in projection and printing apparatus: digital micromirror devices (DMDs) and liquid crystal devices (LCDs).
Texas Instruments has demonstrated prototype projectors using one or more DMDs. DMD devices are described in a number of patents, for example U.S. Pat. Nos. 4,441,791; 5,535,047; 5,600,383 (all to Hornbeck); and U.S. Pat. No. 5,719,695 (Heimbuch). Optical designs for projection apparatus employing DMDs are disclosed in U.S. Pat. Nos. 5,914,818 (Tejada et al.); 5,930,050 (Dewald); 6,008,951(Anderson); and 6,089,717 (Iwai). LCD devices are described, in part, in U.S. Pat. Nos. 5,570,213 (Ruiz et al.) and 5,620,755 (Smith, Jr. et al.).
While there has been some success in color representation using spatial light modulators, there is a long-felt need for a further broadening of the projection color gamut that will enhance special effects and heighten the viewing experience for an audience.
Faced with a similar problem of insufficient color gamut, the printing industry has used a number of strategies for broadening the relatively narrow gamut of pigments used in process-color printing. Because conventional color printing uses light reflected from essentially white paper, the color representation methods for print employ a subtractive color system. Conventionally, the process colors cyan (blue+green), magenta (red+blue) and yellow (red+green) are used for representing a broad range of colors. However, due to the lack of spectral purity of the pigment, combinations of cyan, magenta and yellow are unable to yield black, but instead provide a dark brown hue. To improve the appearance of shadow areas, black is added as a fourth pigment. As is well known in the printing arts, further refined techniques, such as undercolor removal, can then be used to take advantage of less expensive black pigments in full-color synthesis. Hence, today's conventional color printing uses the four color CMYK (Cyan, Magenta, Yellow, and blacK) method described above.
However, even with the addition of black, the range of colors that can be represented by printing pigments is limited. There are specialized colors that cannot be adequately reproduced using the CMYK “process color” system, such as metallic gold or silver, or specific colors such as those used for corporate identity in logos and packaging, for example. To meet this need, a fifth pigment can be added to a selected print run in order to provide “spot color” over specific areas of an image. Using this technique, for example, many companies use special color inks linked to a product or corporate identity and use these colors in packaging, advertising, logos, and the like, so that the consumer recognizes a specific product, in part, by this special color.
Colors in addition to the conventional CMYK process color set have been employed to extend the overall color gamut in printing applications. For example, EP 0 586 139 (Litvak) discloses a method for expanding the conventional color gamut that uses the 4-color CMYK space to a color space using five or more colors.
Referring back to FIG. 1, it is instructive to note that the color gamut is essentially defined by a polygon, where each vertex corresponds to a substantially pure, saturated color source used as a component color. The area of the polygon corresponds to the size of the color gamut. To expand the color gamut requires moving one or more of these vertices closer to the outline of visible gamut 100. Thus, for example, addition of a color that is inside the polygon defining the color gamut does not expand the color gamut. For example, U.S. Pat. No. 5,982,992 (Waldron) discloses using an added “intra-gamut” colorant in a printing application. However, as noted in the specification of U.S. Pat. No. 5,982,992, this method does not expand the color gamut itself, but can be used for other purposes, such as to provide improved representation of pastels or other colors that are otherwise within the gamut but may be difficult to represent using conventional colorants.
Conventional color models, such as the CIE LUV model noted above, represent each individual color as a point in a 3-dimensional color space, typically using three independent characteristics such as hue, saturation, and brightness. Color data, such as conventional image data for a pixel displayed on a color CRT, is typically expressed with 3-color components (for example R, G, B). Conventional color projection film provides images using three photosensitized emulsion layers, sensitive to red, blue, and green illumination. Because of these conventional practices and image representation formats, developers of digital projection systems have, understandably, adhered to a 3-color model. In conformance with conventional 3-color practices and data representation methods, developers of digital projection apparatus have proposed various solutions for illumination systems, such as filtering a bright white light source to obtain red, green, and blue component colors for full color image projection. As just one example, U.S. Pat. No. 6,247,816 (Cipolla et al.) discloses a digital projection system employing dichroic optics to split white light into suitable red, green, and blue color components for modulation.
A few projection solutions have been proposed using more than 3-color light sources. However, the bulk of solutions proposed have not targeted color gamut expansion. Disclosures of projectors using more than 3-color sources include the following:                U.S. Pat. No. 6,256,073 (Pettit) discloses a projection apparatus using a filter wheel arrangement that provides four colors in order to maintain brightness and white point purity. However, the fourth color added in this configuration is not spectrally pure, but is white in order to add brightness to the display and to minimize any objectionable color tint. It must be noted that white is analogous to the “intra-gamut” color addition noted in the printing application of U.S. Pat. No. 5,982,992. As is well known to those skilled in color theory, adding white actually reduces the color gamut.        Similarly, U.S. Pat. No. 6,220,710 (Raj et al.) discloses the addition of a white light channel to standard R, G, B light channels in a projection apparatus. As was just noted, the addition of white light may provide added luminosity and may help to achieve some intra-gamut colors more easily, but this type of solution constricts the color gamut.        U.S. Pat. No. 6,191,826 (Murakami et al.) discloses a projector apparatus that uses four colors derived from a single white light source, where the addition of a fourth color, orange, compensates for unwanted effects of spectral distribution that affect the primary green color path. In the apparatus of U.S. Pat. No. 6,191,826, the specific white light source used happens to contain a distinctive orange spectral component. To compensate for this, filtering is used to attenuate undesirable orange spectral content from the green light component in order to improve the spectral purity of the green illumination. Then, with the motive of compensating for the resulting loss of brightness, a separate orange light is added as a fourth color. The disclosure indicates that some expansion of color range is experienced as a side effect. However, with respect to color gamut, it is significant to observe that the solution disclosed in U.S. Pat. No. 6,191,826 does not appreciably expand the color gamut of a projection apparatus. In terms of the color gamut polygon described above with reference to FIG. 1, addition of an orange light may add a fourth vertex, however, any added orange vertex would be very close to the line already formed between red and green vertices. Thus, the newly formed gamut polygon will, at best, exhibit only a very slight increase in area over the triangle formed using three component colors. Moreover, unless a pure wavelength orange is provided, with no appreciable leakage of light having other colors, there could even be a small decrease in color gamut using the methods disclosed in U.S. Pat. No. 6,191,826.        U.S. Pat. No. 6,280,034 (Brennesholtz) discloses a projection apparatus using up to six colors, employing RGB as well as CMY (cyan, magenta, and yellow) colors that are obtained from a broadband light source. Although such an approach may be useful to enhance brightness and luminance for some colors, the addition of complementary CMY colors does not expand the color gamut and, in practice, could result in a smaller color gamut, as indicated in the disclosure of U.S. Pat. No. 6,280,034. Additionally, the embodiment disclosed in U.S. Pat. No. 6,280,034 uses light sources having different polarizations, which prevents use of an analyzer for improving contrast.        In contrast to the above patent disclosures, U.S. Pat. No. 6,147,720 (Guerinot et al.) discloses a projection system that claims a broadened color gamut by employing six colors. The apparatus disclosed in U.S. Pat. No. 6,147,720 uses a rotating filter wheel to generate red, green, blue, cyan, magenta, and yellow source illumination. Each of these six colors, in sequence, is then directed to a single light valve for modulation. While this apparatus may provide some modest increase in color gamut, there are significant disadvantages to the solution proposed in U.S. Pat. No. 6,147,720. For example, brightness is necessarily constrained whenever a filter wheel is used, due to both the selective action of the filter and to inherent timing and dead time constraints. With a primary color and its complementary color alternated, intensity flicker may be a further problem with the embodiment of U.S. Pat. No. 6,147,720. Continued rapid cycling of arc lamps used as white light sources presents another potential problem source with this design. For these reasons, the apparatus design proposed in U.S. Pat. No. 6,147,720 may not meet performance, reliability, and cost requirements for digital projection.        Patent application Ser. No. WO 01/95544 A2 (Ben-David et al.) discloses a display device and method for color gamut expansion using four or more substantially saturated colors. While the disclosure of application WO 01/95544 provides improved color gamut, however, the embodiments and methods disclosed apply conventional solutions for generating and modulating each color. The solutions disclosed use either an adapted color wheel with a single spatial light modulator or use multiple spatial light modulators, with a spatial light modulator dedicated to each color. When multiplexing a single spatial light modulator to handle more than three colors, a significant concern relates to the timing of display data. The spatial light modulator employed must provide very high-speed refresh performance, with high-speed support components in the data processing path. Parallel processing of image data would very likely be required in order to load pixel data to the spatial light modulator at the rates required for maintaining flicker-free motion picture display. It must also be noted that the settling time for conventional LCD modulators, typically in the range of 10-20 msec for each color, further shortens the available projection time and thus constrains brightness. Moreover, the use of a filter wheel for providing the successive component colors at a sufficiently high rate of speed has further disadvantages. Such a filter wheel must be rotated at very high speeds, requiring a precision control feedback loop in order to maintain precision synchronization with data loading and device modulation timing. The additional “dead time” during filter color transitions, already substantial in devices using 3-color filter wheels, would further reduce brightness and complicate timing synchronization. Coupling the filter wheel with a neutral density filter, also rotating in the light path, introduces additional cost and complexity. Although rotating filter wheels have been adapted for color projection apparatus, the inherent disadvantages of such a mechanical solution are widely acknowledged. Further, without some shuttering means, color crosstalk becomes a problem. Color crosstalk would occur, for example, at a transition of light color while the corresponding data transition is also in process. Alternative solutions using a spatial light modulator dedicated to each color introduce other concerns, including proper alignment for component colors. The disclosure of application WO 01/95544 teaches the deployment of a separate projection system for each color, which would be costly and would require separate alignment procedures for each display screen size and distance. Providing illumination from a single light source results in reduced brightness and contrast. Moreover, the added cost in using four or more spatial light modulators may not justify an incremental improvement in color gamut for consumer projection devices. Thus, while the disclosure of application WO 01/95544 teaches gamut expansion in theory, in practice there are a number of significant drawbacks to the design solutions proposed. As a studied consideration of application WO 01/95544 clearly shows, problems that were difficult to solve for 3-color projection, such as timing synchronization, color alignment, maintaining brightness and contrast, cost of spatial light modulators and overall complexity, are even more challenging when attempting to use four or more component colors.        
Thus, it can be seen that, with respect to projection apparatus, there have been solutions using a fourth color, however, few of these solutions target the expansion of the color gamut as a goal or disclose efficient methods for obtaining an expanded color gamut while maintaining the necessary brightness and overall performance required for digital projection or display. In fact, as is shown in a number of the solutions listed above, there can even be some loss of color gamut with the addition of a fourth color. Proposed solutions for expanding color gamut such as those disclosed in the WO 01/95544 application would be difficult and costly to implement.
Referring back to FIG. 1, it is instructive to note that the broadest possible gamut is achieved when component colors, that is, colors represented by the vertices of the color gamut polygon, are spectrally pure colors. In terms of the gamut mapping of FIG. 1, a spectrally pure color would be represented as a single point lying on the boundary of the curve representing visible gamut 100. As is well known in the optical arts, lasers inherently provide light sources that exhibit high spectral purity. For this reason, lasers are considered as suitable light sources for digital color projection. In some conventional designs, laser beams are modulated and combined and then raster scanned using electromechanical low-speed vertical and high-speed horizontal scanners. These scanners typically comprise spinning polygons for high speed scanning and galvanometer driven mirrors for low speed deflection. Vector scan devices that write “cartoon character” outlines with two galvanometer scanners have long been on the market for large area outdoor laser displays, for example. Lasers have also been used with spatial light modulators for digital projection. As one example, U.S. Pat. No. 5,537,258 (Yamazaki et al.) discloses a laser projection system with red, green, and blue dye lasers providing the primary colors for forming an image using a single shared spatial light modulator.
There have been proposed solutions using more than three lasers within a projector where the additional laser serves a special purpose other than color projection. For example, U.S. Pat. No. 6,020,937 (Bardmesser) discloses a TV display system using as many as four color lasers, however, the fourth laser provides an additional source for achieving increased scan speed and is not a fourth color source. The use of a fourth pump laser is noted in U.S. Pat. No. 5,537,258 cited above and in U.S. Pat. No. 5,828,424 (Wallenstein), which discloses a color projection system that uses a pump laser source with frequency multipliers to excite projection lasers having the conventional R, G, B colors. Again, this use of a fourth laser does not add a fourth projection color.
Unlike color projection film, digital projection presents a full-color image as a composite of individual component color frames, conventionally as red, green, and blue components. A digital projection apparatus, such as that disclosed in U.S. Pat. No. 5,795,047 (Sannohe et al.) may provide all three component color frames simultaneously. However, this method requires three separate spatial light modulators, one dedicated to each color. As a less expensive alternative, a single spatial light modulator can be shared, providing a sequence of component color frames, multiplexed at a rapid rate, so that the human eye integrates separately displayed color frames into a single color image. When using three colors, this multiplexing method may be capable of providing a color-sequenced image in a series of component color frames that are switched rapidly enough so that color transitions are imperceptible to an observer. However, as was noted above with reference to application WO 01/95544, a four-, five-, or six-color projection apparatus may not be able to provide frame sequencing at a sufficient rate for maintaining flicker-free imaging at needed brightness levels. Moreover, at the same time, the added cost of a fourth, fifth, or sixth spatial light modulator may be prohibitive, preventing manufacturers from taking advantage of the additional color gamut that is available.
There have been a number of solutions proposed for reducing the number of spatial light modulators used in a projection apparatus. Field-sequential or color-sequential operation, widely used for low-end projectors such as those used for business presentations, employs a single spatial light modulator that is temporally shared for each of the primary RGB colors, in multiplexed fashion. However, device response time problems for data loading, setup, and modulation response time limit the usefulness of the field-sequential approach for higher quality devices. Proposed alternatives to alleviate response time constraints include configurations using dual spatial light modulators, as in the following examples:                U.S. Pat. No. 6,203,160 (Ho) discloses a projection apparatus using two spatial light modulators, one for modulating the s-polarization component of incident light, the other for modulating the p-polarization component. With a similar approach, U.S. Pat. No. 5,921,650 (Doany et al.) also discloses a projector using two spatial light modulators, one for light having s-polarization and one for light having p-polarization. While the approaches used in U.S. Pat. Nos. 6,203,160 and 5,921,650 provide some advantages with respect to efficient use of light, this type of approach has some drawbacks. Achieving high contrast when using both s- and p-polarization states can be difficult, requiring additional polarization devices in each light modulation path. Both U.S. Pat. Nos. 6,203,160 and 5,921,650 use a broadband white light and a color filter wheel for providing a color illumination source. This approach adds mechanical cost and complexity and limits the flexibility of the illumination system.        U.S. Pat. No. 6,217,174 (Knox) discloses an image display apparatus using two spatial light modulators, with the first spatial light modulator dedicated to a single primary color and the second spatial light modulator multiplexed between the other two primary colors using a color shutter. This approach reduces the switching speed requirements of apparatus using a single spatial light modulator. However, the apparatus disclosed in U.S. Pat. No. 6,217,174, since it is intended for use within a small display device, is designed for a lamp-based light source. It may prove difficult to obtain the necessary brightness or image quality for a projector apparatus using the approach of U.S. Pat. No. 6,217,174, for example. Furthermore, this device has the gamut limitations of three colors derived from a broadband source.        U.S. Pat. Nos. 5,612,753 and 5,905,545 (Poradish et al.) disclose projection apparatus that employ two spatial light modulators, each within a modulator system that has its own projection lens. For providing source illumination, a color filter wheel is deployed in the path of a broadband light source. The approach disclosed in U.S. Pat. Nos. 5,612,753 and 5,905,545 alleviates the timing constraints of projection apparatus when compared against approaches using a single spatial light modulator in field sequential fashion. However, the arrangement of components disclosed in these patents is mechanically complex, requires multiple separate projection optics and, because it derives color illumination from a broadband light source, is limited with respect to brightness. The apparatus disclosed in U.S. Pat. No. 6,280,034 (Brennesholtz) described above utilizes dual spatial light modulators, one for RGB primary colors, the other for CMY complementary colors. As was noted, this approach augments the luminance range available, rather than expanding the color gamut. Moreover, with this arrangement, both spatial light modulators operate in color sequential mode, each shared among three colors in multiplexed fashion. Thus, the arrangement of U.S. Pat. No. 6,280,034 provides no relief for timing problems due to color sequential operation when compared with existing three-color projection solutions.        
Thus, although there have been some proposed solutions using two or more spatial light modulators for projection apparatus that use three or more colors, there is room for improvement. Lamps and other broadband light sources set practical limits on brightness levels achievable, particularly where color filter wheels or similar devices that cause some amount of light attenuation or have inherent “dead space” during transitions are employed. The use of color wheels makes it unwieldy to alter or adjust illumination timing. Response times of spatial light modulator devices further constrain the possible timing sequences, particularly where these devices are multiplexed among three colors. In the face of these difficulties, the advantages of expanding the color gamut with an additional color would not be considered within reach using conventional design approaches.
Thus, it can be seen that although conventional approaches for digital projection can be used for a projection or display system using six colors, there is a need for an effective solution that provides the benefits of increased color gamut at reasonable cost, without sacrificing brightness and overall image quality.